1. Field of the Invention
The present invention relates to the electronic sensing of shallow buried objects in the ground surface over large areas, and more particularly to sensors and systems able to look past the strong signal reflections bouncing back from the surface.
2. Description of the Problems to be Solved
Various kinds of conventional, mobile, portable equipment and methods are in use worldwide to scan the top layers and surfaces of the ground to detect and locate valuable, and sometimes dangerous objects. Meteorite hunters routinely use tuned coils sensitive to magnetic materials to sweep for melted and burnt bits of iron and steel that fall to the ground from meteors. Other kinds of metal detectors and ground scanners are used by treasure hunters to find hordes of gold, silver, copper and other precious metals and artifacts.
Radar based detectors and active sonars universally send either continuous wave (CW) or pulsed transmissions into the ground so that echoes returned from buried objects can be received and interpreted. A few synthetic aperture radars use multi-element linear antenna arrays to improve resolution.
A problem common to all conventional radars and active sonars is the so-called “first-interface” between the atmosphere and the top surface of the ground that returns a very strong reflection. Such first-interface reflections can be so strong as to completely overwhelm and push aside the much fainter signals being reflected and returned by small, shallow objects, and especially from targets very deeply buried. Often the reflective things at the surface are of no interest, and they can change their character with wind, rain, road travel, and benign human activity, making for too many false positives.
Construction crews seem to be routinely ripping accidently into buried wires, pipes, and vaults in spite of many official programs and laws for them not to dig without checking first. Sometimes these failures are harmless and little damage is done, but other times serious and catastrophic breaches can occur that take lives, inflict injuries, and costs millions of dollars.
Not every bit of our infrastructures are fully mapped and known, the smaller, older industrial bits have often escaped being inventoried, cataloged and registered. The problems are especially pronounced in third world countries and war torn areas subject to quick fixes and unauthorized construction.
Pipes, wires, canisters, tubs, and other manufactured items are made of a variety of materials that will electromagnetically or dielectrically contrast with surrounding soils if buried in the earth. The things we are concerned with here are not so deeply buried, it is practical to scan them with detectors, even on-the-fly.
In some applications, what's of interest is target objects that were not present before and have recently appeared. In other applications what has recently appeared is of no interest and can be ignored. In still other applications, only manmade objects are of interest.
The trouble is many naturally occurring things can appear to a radar or sonar scan to be manmade objects. For example, a heavy overnight dew can produce puddles of conductive water that will contrast with a more dielectric soil. The shapes these can take often mimic particular objects of interest.
The present inventors discovered that resonant microwave patch antennas (RMPA) driven by continuous wave signals kept at resonance can be used as a very sensitive sensor. Changes in the dielectric character of the immediate environment will affect the loading on the RMPA and therefore manifest as changes in its complex input impedance at resonance. Movements and stationary anomalies deep in the ground can be sensed and characterized.
Various kinds of conventional, mobile, portable equipment and methods are in use worldwide to scan the top layers and surfaces of the ground to detect and locate valuable, and sometimes dangerous objects. Meteorite hunters routinely use tuned coils sensitive to magnetic materials to sweep for melted and burnt bits of iron and steel that fall to the ground from meteors. Other kinds of metal detectors and ground scanners are used by treasure hunters to find hordes of gold, silver, copper and other precious metals and artifacts.
Radar based detectors universally send either continuous wave (CW) or pulsed radio transmissions into the ground so that echoes returned from buried objects can be received and interpreted. As we have described in several of our earlier United States patents, the so-called “first-interface” between the atmosphere and the top surface of the ground will return a very strong reflection. Such first-interface reflections can completely swamp and obscure the much fainter signals being reflected by small, shallow objects, and/or ones very deeply buried.
Manufactured objects buried in the ground can be constructed as all-plastic, low-metal, and all-metal. The smaller they are overall, of course, the more difficult they will be to detect because they return fainter signals. All-plastic and low-metal objects can avoid or frustrate detection by conventional methods that depend on the presence of iron and concomitant electromagnetic response.
Sensors deployed to find these more common objects therefore need to operate in many kinds of modes. The dielectric contrasts of these devices with the surrounding soils can be used to advantage to highlight the object for analysis of signature characteristics.
One-pass and two-pass methods have been conventionally used to find wires, pipes, and other objects-of-interest. One-pass methods must be used when there has been no previous opportunity to make and record a prior sweep or survey.
Two-pass methods provide more useful results, but only if the second pass matches the course of the first pass. A preliminary sweep of the ground must be collected to determine one or more baseline conditions. Rugged high capacity storage media is getting very inexpensive, making large surveys very practical and affordable. New data from subsequent sweeps can then be advantageously compared to data from the priors sweeps to highlight any changes.
Getting the data between passes to be coherent and register properly is not so easy. Practical, real world coherent change detection (CCD) processing of GPR signals obtained from multiple passes and scans is described in more detail in United States Patent Application US 2013-0050008, published by Robert Atkins, et al., Feb. 28, 2013.
Many changes can be quite benign or of no interest whatsoever, e.g., puddles from a recent rain, morning dew, roadside debris, construction, ruts, footprints, and other ordinary events. Very few changes that get noticed will actually signal something of interest has been detected. Often what is of interest are things that were deliberately placed in the interim, and such indications are too important to be missed or misinterpreted.
There is a need for a device and system that can electronically detect and characterize shallow things near the ground surface without any false positives or negatives.